Turning is machining performed by rotating a workpiece attached to a main shaft and pressing a cutting tool attached to a tool post against the workpiece. When the workpiece is long, a minute distortion may be generated in the workpiece due to the machining load caused by the cutting tool being pressed against the workpiece and thus the cylindricity may be reduced. In view of this problem, there is a technology in which, when machining is performed by using a machine tool having upper and lower tool posts, balance cutting, in which the cutting tools are pressed against a workpiece from the upper and lower sides, is performed, whereby the machining loads caused by the cutting tools being pressed against the workpiece are canceled out and the cylindricity is improved (for example, see Patent Literature 1).
Moreover, with this technology, because the machining loads from the upper and lower tool posts can be made equal by performing balance cutting, the cutting feed speed can be doubled and thus the machining time can be shortened.